This invention relates generally to the field of data handling devices, and more particularly, but not by way of limitation, to a method and apparatus for improving servo performance in a disc drive by reducing the effects of repeated runout head position error.
Disc drives are data handling systems used to store and retrieve digital data. A typical disc drive comprises one or more rigid magnetic storage discs which are arranged about a spindle motor for rotation at a constant high speed. A corresponding array of read/write heads are provided to transfer data between tracks defined on the disc surfaces and a host device (such as a computer) in which the disc drive is mounted.
The heads are mounted to a rotary actuator assembly and are controllably positioned adjacent the tracks by a closed loop servo control system. The actuator includes an actuator motor (such as a voice coil motor, VCM) and one or more actuator arms which support the heads over the disc surfaces. The servo control system applies currents to the VCM to move the heads in response to detected and estimated positions of the heads as well as command inputs indicating desired positions of the heads. During operation, the servo control system generates a position error signal (PES) which provides an indication of the intra-track location of a selected head with respect to an associated track over which the head is disposed.
Servo control systems are designed to provide stable positional control in the presence of different types of disturbances which can adversely impact the ability of the disc drive to access and follow a particular track. Such disturbances include externally generated vibrations which are applied to the disc drive housing from the environment in which the disc drive is mounted, internally generated vibrations induced by the rapid movement of the actuator assembly during a seek operation as a head is moved from one track to another, errors in the locations of servo data used to define the tracks which can introduce small, yet significant, track eccentricities, spindle motor vibrations at bearing cage frequencies, and disc slippage which can introduce track eccentricity at the frequency of disc rotation. Such disturbances can be categorized as either repeatable or nonrepeatable runout errors (RRO and NRRO, respectively), and are typically manifested as frequency components of the PES.
Generally speaking, RRO errors are repetitive in nature (usually over each disc revolution) whereas NRRO errors occur more or less randomly over time. The prior art is replete with various approaches to compensating for each of these types of disturbance. Clearly, such disturbances become increasingly adverse as track densities and data transfer rates increase. Disturbances that may have only had a minimal effect upon servo performance on drives just a few years ago can now have a significant impact upon drives of the present generation.
Of particular interest to the present discussion are disturbances of the RRO variety, which as mentioned above can arise from a number of sources such as disc slippage and servo data location errors. Prior art approaches to compensating for such RRO effects have typically involved the evaluation of the PES over a number of disc revolutions to generate a series of compensation values which are then stored in a compensation table and thereafter fed forward into the servo loop during operation. This approach is exemplified by U.S. Pat. No. 4,594,622 issued to Wallis and U.S. Pat. No. 5,539,714 issued to Andrews et al. While operable, such an approach becomes increasingly difficult to implement for higher performance drives. The use of adaptive tables resident in memory imposes a latency cost to access and retrieve a value for each servo sample (e.g., at each servo interrupt). This cost becomes increasingly burdensome at higher servo sample rates and with the compensation of larger numbers of harmonics.
Accordingly, there remains a continued need for improvements in the art to compensate for repeated runout position error in a disc drive servo system, and it is to such improvements that the present invention is directed.
The present invention is directed to an apparatus and method for compensating repeated (persistent) runout position error in a disc drive data handling system.
In accordance with preferred embodiments, the disc drive handling system includes an actuator assembly having an actuator arm which supports a head adjacent a recording surface. A servo circuit includes a servo controller which controls position of the head in response to a position error signal indicative of position of the head with respect to the recording surface.
The servo circuit further includes a filter operably coupled in parallel with the servo controller to receive the position error signal and to generate a compensation signal to reduce the effects of the repeated runout head position error. The compensation signal is based on the position error signal and a frequency of rotation of the recording surface.
The filter is configured to provide a notch in an error sensitivity function relating the position error signal to the repeated runout position error, with the notch being nominally centered at the desired attenuation frequency, such as a multiple of the spindle motor rotation frequency. The filter advantageously operates to calculate the compensation signal values on-the-fly, thereby eliminating the need to store and retrieve different compensation values in memory for each position error signal sample over a complete rotation of the recording surface. Thus, the filter is capable of providing compensation at higher servo sample rates and for larger numbers of harmonics without latency and access costs associated with the prior art.
These and various other features as well as advantages which characterize the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.